(1) Field of the Invention
The present invention relates to a duplex system having a function to suppress unnecessary radio wave radiation, and more particularly to a duplex system comprising a plurality of cards each having electronic parts mounted thereon, a backboard supporting the plurality of cards standing close together in parallel, a plurality of clock signal lines wires closely in each card, and duplex clock signal sources for outputting the duplex clock signals.
In recent years, a method of reducing Electro Magnetic Interference (EMI) for suppressing the radiation of unnecessary electromagnetic waves has been desired. This is because radio wave radiated from electronic devices and their peripheral devices induce noises in other electronic devices so that erroneous operations will occur in these other electronic devices.
To prevent these erroneous operations, rules have been established in some countries such as in the United States of America and in European countries, or voluntary controls are imposed in certain industrial fields, as conducted in Japan. Under these rules or restrictions, the radiation limit of the radio wave is practically determined.
The present invention relates to the suppression of unnecessary radio wave radiation as described above.
(2) Description of the Related Art
In a conventional duplex system comprising a plurality of cards each having electronic parts mounted thereon, a backboard for supporting the plurality of cards standing close together in parallel, a plurality of clock signal lines wired closely in parallel on the backboard for supplying clock signals to each card, and duplex clock signal sources for outputting the duplex clock signals, there is no countermeasure to suppress unnecessary radiation of radio waves so that the unnecessary radiation of radio waves is generated in an amount over that allowed by the above-mentioned rules or restrictions.
In particular, when a high frequency clock signal of, for example, several mega hertz to several tens of megahertz is employed, its higher harmonics are easily radiated from the backboard or the cards.